Hard tissues constituting the human body are composed of 67 wt % inorganic material and 33 wt % organic material, and the inorganic material is composed of apatite, the major ingredient of which is Ca/P. Since the composition ratio of inorganic material is high, substitution is possible by using synthetic hydroxyapatite when a hard tissue defect occurs. However, synthetic hydroxyapatite has a disadvantage in that it is difficult to apply to various sites requiring mechanical stability due to its low compressive strength.
Hydroxyapatite is a representative osteoconductive material which directly attaches to bone when it makes its contact with a peripheral defect in graft sites. The material causes bone fusion when it is in exact contact with a defect site. However, if the material is not in contact with the defect site, it may damage surrounding bones by micro-motion of a graft material. Accordingly, the design thereof can be tailored exactly to the graft site in order to enhance a therapeutic effect.
The characteristics of typical ceramic materials are excellent in compressive strength, hardness, and abrasion resistance. Further, since ceramic materials exist in the form of oxides, they have excellent resistance to corrosion caused by chemical substances. The excellent physical/chemical stability of ceramic materials has the advantage of significantly increasing life expectancy when used in various parts. However, due to the rigidity of the materials, processing thereof is difficult, and as a result, the preparation of a complex molded product is not possible.
Since the rigidity of ceramic materials appears due to heat treatment, e.g., sintering, it is common to complete the molding before the heat treatment, and after the heat treatment, some of the materials are processed using a polishing method. Ceramic molded products can be produced by simply applying pressures to powders before sintering, or by making the powder into a slurry state and then casting, injection molding, or extrusion molding the same. The molded products produced as above are treated with heat at the sintering temperature of raw materials, and the preparation is thereby completed. When molding a product design before heat treatment, a mold implementing the same is required, and the mold can only be used for production of the corresponding design.
When various designs of products are implemented, it is necessary to apply the processing after heat treatment. Due to the characteristics of ceramics, such method is disadvantageous in that it requires considerable time and costs. In addition, as the high hardness of the surface is likely to cause defects during impractical processing, a defect rate may be increased due to the processing. If damage occurs due to a processing defect after grafting the same into a human body, not only is the graft site severely damaged, but also re-operation should be carried out.
Therefore, when preparing a material for replacing a hard tissue, it is essential to produce a material which is tailored to fit the defect site exactly.